ACPI: processor: refactor internal map_lsapic_id()

Un-nest the if statements for readability.

Remove comments that re-state the obvious.

Change the control flow so that we no longer need a temp variable.

Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>